Moderator is a material which is used to slow down the neutrons produced during nuclear fission. The neutrons from the source are of high speed and energy. Heavy water or graphite moderators slow down the speed of the neutrons. The energy of fast moving neutrons decreases from 2MeV to 0.02535 eV, it corresponds to the velocity of 220 m `"sec"^(-1)`. At this velocity, the neutrons are in thermal equilibrium with the moderator. such neutrons are called thermal neutrons. Thermal neutrons cause further fission reaction. The essential characterstices of moderators are:
(i) Its molar mass must be low,
(ii) It should not absorb neutrons.
(iii) It should undergo elastic collisions with neutrons.
Which of the following is not used as a moderator?
A. Heavy water
B. Graphite
C. Beryllium
D. Sodium
